This invention relates generally to aircraft turbine engines, and more specifically, to propulsion systems used with a multiple turbine engine assembly.
In some known aircraft, propulsion systems are used to control a flow of exhaust gases for a variety of aircraft functions. For example, such systems can be used to provide thrust for Vertical Take-Off and Landing (VTOL), Short Take-Off Vertical Landing (STOVL) and/or Extreme Short Take-Off and Landing (ESTOL) aircraft. At least some known STOVLs and ESTOLs use vertical thrust posts to facilitate short and extremely short take-offs and landings. In aircraft using vertical thrust posts or nozzles, exhaust from a common plenum is channeled to thrust posts during take-off and landing operations, and, at a predetermined altitude, through a series of valves, the exhaust is channeled from the common plenum to a cruise nozzle.
Other known STOVLs and ESTOLs use rotor tip gas reaction driven rotors and rotor tip driven rotors to facilitate vertical take-offs and landings. During flight, some such rotor-driven aircraft can transform into fixed wing aircraft. In some of such aircraft, the engine exhaust gases may also be used to control yaw through aft mounted variable area exhaust nozzles. Other known aircraft use the propulsion system to reduce drag on the aircraft, and/or cool aircraft wings. In such aircraft, gases are channeled through the wings of the aircraft and discharge through a plurality of openings defined in a trailing portion of the wing. Such aircraft are sometimes referred to as having “blown wings”.
Because single engine assemblies may limit the survival capabilities of the aircraft when engine power loss occurs, each of the above described propulsion systems may be preferred to be used with a multiple, especially a dual, engine assembly. In a single engine assembly, the engine exhausts into a plenum where it is then channeled via the propulsion system for use by the aircraft. In a dual engine propulsion control system, each engine exhausts into a common plenum wherein the exhaust is channeled for use by the aircraft. However, because the exhausts are mixed in a common plenum, if one engine stalls, fails, or is in any way rendered inoperable, the exhaust flow from that engine may not enter the plenum. The associated pressure drop in the plenum may adversely affect the operation of the propulsion system and valves must be provided to prevent back flow of exhaust gases into the inoperable engine.
A control system must also be provided to actuate these valves at a high slew rate to prevent the operable engine from stalling. Such a control system capability has been envisioned but is not known to have been demonstrated. Such a control system would be required to make major airflow adjustments in the propulsion system that may include closing valves to prevent loss of plenum pressure through the inoperable engine and to attempt to redirect nozzle areas to facilitate retaining flight stability. Such significant readjustments of nozzle area configuration may lead to the stall of the operational engine and may cause a complete loss of lift. Furthermore, it may be difficult for the control system to increase the power setting of the operable engine to an emergency level.
Because of the time and cost of developing a control system and implementing the mechanical complexity created by such propulsion systems having multiple engines exhaust into a common plenum, such propulsion systems may be much more costly than those used with single engine assemblies. For example, to provide exhaust control, such propulsion systems include a plurality of sensors and valves that can be selectively actuated to isolate an inoperable engine. More specifically, known propulsion systems used with dual engine assemblies include a plurality of butterfly valves and actuation systems that direct the exhaust gases in the event of an engine power loss. The valves enable an inoperable engine to be isolated to prevent back-flow through the inoperative engine. In addition, the isolation valves enable emergency flight capability and limited aircraft control with an inoperable engine. However, because such valves must operate at high temperatures, such valves and associated actuation systems may require large actuation forces. Furthermore, in known propulsion systems, once the valves have been closed to isolate an inoperable engine, it is not possible to attempt to restart an inoperable engine.